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Euastrum didelta Desmid Species Outer Hebrides

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Phylum: Charophyta   Family: Desmidiaceae

Euastrum didelta J. Ralfs 1848

Semicells pyramidal with simple basal lobes and a lateral lobe above the median line. Apices truncate with a deep, closed, median incision. In face view there are 5 protuberances, three across the base of the semicell, and two across the centre. At the base of these two is a noticeable scrobiculum.
Cell dimensions: L: 121-156 µm; B: 67-81 µm; Is: 19-25 µm; Th: 42-45; L/B: 1.70-1.94.
West & West (1905: 17) note: “This is one of the most abundant of the large British species of Euastrum and the subject of considerable variation in outward form.” This is very similar to E. humerosum, the main difference is in the basal angle: entire in E. didelta, geminate in E. humerosum. I wonder how many of the early records have been misidentified because it’s rare in the Outer Hebrides, which has ample suitable habitat. An acidophile of moorland pools and bogs.

References: 
Coesel, P.F.M. & Meesters, K.J. (2023 second edition) Desmids of the Lowlands.
Ralfs, J. (1848) The British Desmidieae.
Ruzicka, J. (1981) Die Desmidiaceen Mitteleuropas, Volume 1, Part 2.
West, W. & West, G.S. (1905) A Monograph of the British Desmidiaceae, Volume 2.

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